These are rather bizarre objects.
Quoting from the article :
"We still have no idea how big or far away they are. They could be objects in our galaxy, perhaps a few light-years across, or they could be far away in the Universe and maybe millions of light years across."
That is even more bizarre!
Since these are such strange objects, and they are not explained by the most obvious sources, it is time for some WAGs, since wild notions clearly cannot be ruled out.
It seems possible that an ORC is the result of a photon sphere (1), which is a large number of captured photons orbiting, in this case, a massive black hole. The photons are too far away to be pulled into the BH, but the gravitational field is strong enough to prevent their escape from orbiting. The light from these object seen in telescopes is somehow generated by the photon sphere, and that light can escape since it is not bound to the sphere.
Or perhaps the captured light is slowly being released from the sphere as the BH evaporates! Surely this is a WAG, but one that fits the limited data.
1.
https://en.wikipedia.org/wiki/Photon_sphere